Semiconductor structure and manufacturing method thereof

ABSTRACT

A semiconductor structure includes a substrate, a first gate structure, and a second gate structure. The substrate has a plurality of first fins and a plurality of second fins, wherein a first pitch between two adjacent first fins is greater than a second pitch between two adjacent second fins. The first gate structure crosses over the first fins. The second gate structure crosses over the second fins, wherein the second gate structure includes an upper portion having two first sidewalls substantially parallel to each other and a lower portion tapers toward the substrate.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims priority of U.S. provisional application Ser. No. 62/490,020, which was filed on Apr. 25, 2017, the entire disclosure of which is incorporated herein by reference.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapid growth and has strived for higher device density, higher performance, and lower costs. However, problems involving both fabrication and design have been encountered. One solution to these problems has been the development of a fin-like field effect transistor (FinFET). A FinFET includes a thin vertical ‘fin’ formed in a free standing manner over a major surface of a substrate. The source, drain, and channel regions are defined within this fin. The transistor's gate wraps around the channel region of the fin. This configuration allows the gate to induce current flow in the channel from three sides. Thus, FinFET devices have the benefit of higher current flow and reduced short-channel effects.

The dimensions of FinFETs and other metal oxide semiconductor field effect transistors (MOSFETs) have been progressively reduced as technological advances have been made in integrated circuit materials. For example, high-k metal gate (HKMG) processes have been applied to FinFETs.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1-8, 10, 12, 14, 16, 18, and 20 illustrate a method for manufacturing a semiconductor structure at various stages in accordance with some embodiments of the instant disclosure.

FIGS. 9A, 11A, 13A, 15A, 17A, 19A, and 21A are cross-sectional views along lines A-A′ in FIGS. 8, 10, 12, 14, 16, 18, and 20 respectively.

FIGS. 9B, 11B, 13B, 15B, 17B, 19B, and 21B are cross-sectional views along lines B-B′ in FIGS. 8, 10, 12, 14, 16, 18, and 20 respectively.

FIGS. 9C, 11C, 13C, 15C, 17C, 19C, and 21C are cross-sectional views along lines C-C′ in FIGS. 8, 10, 12, 14, 16, 18, and 20 respectively.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

The fins may be patterned by any suitable method. For example, the fins may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. Generally, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in one embodiment, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned sacrificial layer using a self-aligned process. The sacrificial layer is then removed, and the remaining spacers may then be used to pattern the fins.

FIGS. 1-8, 10, 12, 14, 16, 18, and 20 illustrate a method for manufacturing a semiconductor structure at various stages in accordance with some embodiments of the instant disclosure.

Reference is made to FIGS. 1-2. A substrate 110 having a plurality of first fins 110 a and a plurality of second fins 110 b is formed. The substrate 110 shown in FIG. 1 is illustrated. The substrate 110 has a core region CR and a periphery region PR adjacent to the core region CR. For example, the periphery region PR surrounds the core region CR. The periphery region PR can be referred as an input/output (I/O) region. In some embodiments, the substrate 110 may be a semiconductor substrate, such as a bulk semiconductor, a semiconductor-on-insulator (SOI) substrate, or the like. The substrate 110 may be a wafer, such as a silicon wafer. An SOI substrate includes a layer of a semiconductor material formed on an insulator layer. The insulator layer may be, for example, a buried oxide (BOX) layer, a silicon oxide layer, or the like. The insulator layer is provided on a substrate, a silicon substrate or a glass substrate. Other substrates, such as a multi-layered or gradient substrate may also be used. In some embodiments, the semiconductor material of the substrate 110 may include silicon; germanium; a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof.

Please still referring to FIG. 1, a plurality of first photo-sensitive layers 120 are formed on the core region CR of the substrate 110, and a plurality of second photo-sensitive layers 130 are formed on the periphery region PR of the substrate 110. Although two first photo-sensitive layers 120 and two second photo-sensitive layers 130 are shown in FIG. 1, it should be clear that there are number of such “photo-sensitive layers” that are separated from one another. Moreover, it is noted that a first pitch p₁₁ between two adjacent first photo-sensitive layers 120 is greater than a second pitch p₁₂ between two adjacent second photo-sensitive layers 130. In some embodiments, a width w₁₁ of the first photo-sensitive layer 120 is substantially equal to a width w₁₁ of the second photo-sensitive layer 130. However, it is noted that the scope of this application is not limited thereto. The term “substantially” as used herein may be applied to modify any quantitative representation which could permissibly vary without resulting in a change in the basic function to which it is related.

In some embodiments, the photo-sensitive layers 120 and the second photo-sensitive layers 130 are formed by the following operations. A photo-sensitive layer is formed on the core region CR and the periphery region PR of the substrate 110. The photo-sensitive layer is patterned, forming openings in the photo-sensitive layer, so that some regions of the substrate 110 are exposed, and thus the first photo-sensitive layers 120 and the second photo-sensitive layers 130 are formed.

Please referring to FIG. 2, the substrate 110 is etched through the first photo-sensitive layers 120 and the second photo-sensitive layers 130 to form the first fins 110 a, the second fins 110 b, first trenches T1 adjacent to the first fins 110 a, and second trenches T2 adjacent to the second fins 110 b. More specifically, a portion of the substrate 110 between neighboring first trenches T1 forms the first fin 110 a, and a portion of the substrate 110 between neighboring second trenches T2 forms the second fin 110 b. The first trenches T1 and the second trenches T2 may be trench strips (when viewed in the top view of the semiconductor structure) that are substantially parallel to each other. In some embodiments, a height of the first fin 110 a is substantially the same as a height of the second fin 110 b. Although two first fins 110 a and two second fins 110 b are shown, it should be clear that there are number of such “fins” that are separated from one another. In some other embodiments, the numbers of the first fins 110 a and the second fins 110 b can be different. For example, the number of the second fins 110 b is greater than the number of the first fins 110 a. The number of the first fins 110 a can be less than or equal to 4, and the number of the second fins 110 b can be greater than or equal to 12.

Because the substrate 110 is patterned by the first photo-sensitive layers 120 and the second photo-sensitive layers 130, a first pitch p₂₁ between two adjacent first fins 110 a is greater than a second pitch p₂₂ between two adjacent second fins 110 b. In some embodiments, a fin width w₂₁ of the first fin 110 a is substantially equal to a fin width w₂₂ of the second fin 110 b. However, it is noted that the scope of this application is not limited thereto. In some other embodiments, the fin width w₂₁ of the first fin 110 a is substantially greater than the fin width w₂₂ of the second fin 110 b. In still some embodiments, the fin width w₂₁ of the first fin 110 a is substantially less than the fin width w₂₂ of the second fin 110 b. After forming the first fins 110 a and the second fins 110 b, the first photo-sensitive layers 120 and the second photo-sensitive layers 130 are removed.

Attention is now invited to FIG. 3. An isolation dielectric 140 is formed to cover the first fins 110 a and the second fins 110 b. The isolation dielectric 140 has a thickness t₁. The isolation dielectric 140 may overfill the first trenches T1 and the second trenches T2. The isolation dielectric 140 in the first trenches T1 and the second trenches T2 can be referred to as shallow trench isolation (STI) structure. In some embodiments, the isolation dielectric 140 is made of silicon oxide, silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), or other low-K dielectric materials. In some embodiments, the isolation dielectric 140 may be formed using a high-density-plasma (HDP) chemical vapor deposition (CVD) process, using silane (SiH₄) and oxygen (O₂) as reacting precursors. In some other embodiments, the isolation dielectric 140 may be formed using a sub-atmospheric CVD (SACVD) process or high aspect-ratio process (HARP), wherein process gases may include tetraethylorthosilicate (TEOS) and ozone (O₃). In yet other embodiments, the isolation dielectric 140 may be formed using a spin-on-dielectric (SOD) process, such as hydrogen silsesquioxane (HSQ) or methyl silsesquioxane (MSQ). Other processes and materials may be used. In some embodiments, the isolation dielectric 140 can have a multi-layer structure, for example, a thermal oxide liner layer with silicon nitride formed over the liner. Thereafter, a thermal annealing may be optionally performed to the isolation dielectric 140.

Reference is made to FIG. 4. The thickness t₁ of the isolation dielectric 140 is reduced to leave the isolation dielectric 140 covering top surfaces TS1 of the first fins 110 a and top surfaces TS2 of the second fins 110 b. In other words, reducing the thickness t₁ of the isolation dielectric 140 stops before the top surfaces TS1 of the first fins 110 a and the top surfaces TS2 of the second fins 110 b expose. In other words, a portion of the isolation dielectric 140 outside the first trenches T1 and the second trenches T2 is removed without exposing the first fins 110 a and the second fins 110 b. The thickness t₁ of the isolation dielectric 140 shown in FIG. 3 is reduced to a thickness t₂ of the isolation dielectric 140 shown in FIG. 4. In some embodiments, reducing the thickness t₁ of the isolation dielectric 140 is performed by chemical-mechanical planarization (CMP).

Please refer to FIG. 5. The isolation dielectric 140 covering the top surfaces TS1 of the first fins 110 a and the top surfaces TS2 of the second fins 110 b is etched to form a first isolation dielectric 140 a between the first fins 110 a and a second isolation dielectric 140 b between the second fins 110 b. More specifically, etching the isolation dielectric 140 covering the top surfaces TS1 of the first fins 110 a and the top surfaces TS2 of the second fins 110 b stops until the top surfaces TS1 and sidewalls SW1 of the first fins 110 a and the top surfaces TS2 and sidewalls SW2 the second fins 110 b are exposed. In other words, etching the isolation dielectric 140 covering the top surfaces TS1 of the first fins 110 a and the top surfaces TS2 of the second fins 110 b includes removing the isolation dielectric 140 above the top surfaces TS1 of the first fins 110 a and the top surfaces TS2 of the second fins 110 b, a portion of the isolation dielectric 140 between the first fins 110 a, and a portion of the isolation dielectric 140 between the second fins 110 b. After etching the isolation dielectric 140, a portion of the first fin 110 a is higher than a top of the first isolation dielectric 140 a, and a portion of the second fin 110 b is higher than a top of the second isolation dielectric 140 b. Hence, this portion of the first fin 110 a protrudes above the first isolation dielectric 140 a, and this portion of the second fin 110 b protrudes above the second isolation dielectric 140 b. In some embodiments, etching the isolation dielectric 140 is performed by dry etching, wherein diluted HF, SiCoNi (including HF and NH₃), or the like, may be used as the etchant.

As shown in FIG. 5, it is noted that the first isolation dielectric 140 a between the first fins 110 a is thinner than the second isolation dielectric 140 b between the second fins 110 b. Accordingly, the top of the first isolation dielectric 140 a is below the top of the second isolation dielectric 140 b. Moreover, a first height h₁ from the top of at least one of the first fins 110 a to the top of the first isolation dielectric 140 a is greater than a second height h₂ from at least one of the top of the second fins 110 b to the top of the second isolation dielectric 140 b. In some embodiments, a height difference between the first height h₁ and the second height h₂ (h₁−h₂) is greater than about 3 nm. In some embodiments, the first fins 110 a substantially level with the second fins 110 b.

Reference is made to FIG. 6. A gate dielectric layer 150 is blanket formed over the first fins 110 a, the second fins 110 b, the first isolation dielectric 140 a, and the second isolation dielectric 140 b. After the gate dielectric layer 150 is formed, a dummy gate electrode layer 160 is formed over the gate dielectric layer 150. In some embodiments, the gate dielectric layer 150 is made of high-k dielectric materials, such as metal oxides, transition metal-oxides, or the like. Examples of the high-k dielectric material include, but are not limited to, hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO), hafnium tantalum oxide (HMO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), zirconium oxide, titanium oxide, aluminum oxide, hafnium dioxide-alumina (HfO₂-A1 ₂O₃) alloy, or other applicable dielectric materials. In some embodiments, the gate dielectric layer 150 is an oxide layer. The gate dielectric layer 150 may be formed by a deposition processes, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), plasma enhanced CVD (PECVD) or other suitable techniques. In some embodiments, the dummy gate electrode layer 160 may include polycrystalline-silicon (poly-Si), poly-crystalline silicon-germanium (poly-SiGe), metallic nitrides, metallic silicides, metallic oxides, or metals. In some embodiments, the dummy gate electrode layer 160 includes a metal-containing material such as TiN, TaN, TaC, Co, Ru, Al, combinations thereof, or multi-layers thereof. The dummy gate electrode layer 160 may be deposited by CVD, physical vapor deposition (PVD), sputter deposition, or other techniques suitable for depositing conductive materials.

For clarity, in the following FIGS. 7-17C, the gate dielectric layer 150 over the core region CR is referred to as a gate dielectric layer 150 a. The dummy gate electrode layer 160 over the core region CR is referred to as a dummy gate electrode layer 160 a. The gate dielectric layer 150 over the periphery region PR is referred to as a gate dielectric layer 150 b. The dummy gate electrode layer 160 over the periphery region PR is referred to as a dummy gate electrode layer 160 b.

Please refer to FIGS. 7-8. The dummy gate electrode layers 160 a, 160 b, and the gate dielectric layers 150 a, 150 b, are etched to form a plurality of first dummy gate structures DGS1 and a plurality of second dummy gate structures DGS2. At least one of the first dummy gate structures DGS1 includes the gate dielectric layer 150 a and the dummy gate electrode layer 160 a, and crosses the first fins 110 a. At least one of the second dummy gate structures DGS2 includes the gate dielectric layer 150 b and the dummy gate electrode layer 160 b, and crosses the second fins 110 b.

As shown in FIG. 7, a plurality of first mask layers 170 are formed on the dummy gate electrode layer 160 a, and a plurality of second mask layers 180 are formed on the dummy gate electrode layer 160 b. In FIG. 7, a width w₃₁ of at least one of the first mask layers 170 is less than a width w₃₂ of at least one of the second mask layers 180. However, it is noted that the scope of this application is not limited thereto. In some other embodiments, a width w₃₁ of at least one of the first mask layers 170 is substantially equal to a width w₃₂ of at least one of the second mask layers 180. In still some embodiments, a width w₃₁ of at least one of the first mask layers 170 is greater than a width w₃₂ of at least one of the second mask layers 180. The first mask layers 170 and second mask layers 180 may be hard masks for protecting the underlying dummy gate electrode layer 160 a, 160 b, and the gate dielectric layer 150 a, 150 b, against subsequent etching process. The first mask layers 170 and second mask layers 180 may be formed by a series of operations including deposition, photolithography patterning, and etching processes. The photolithography patterning processes may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), and/or other applicable processes. The etching processes may include dry etching, wet etching, and/or other etching methods (e.g., reactive ion etching).

As shown in FIG. 8, the dummy gate electrode layer 160 a and the gate dielectric layer 150 a is etched through the first mask layers 170 to form the first dummy gate structures DGS1 wrapping portions of the first fins 110 a, and the dummy gate electrode layer 160 b and the gate dielectric layer 150 b is etched through the second mask layers 180 to form the second dummy gate structures DGS2 wrapping portions of the second fins 110 b. At least one of the first dummy gate structures DGS1 includes a portion of the dummy gate electrode layer 160 a and a portion of the gate dielectric layer 150 a covered and protected by the first mask layer 170. At least one of the second dummy gate structures DGS2 includes a portion of the dummy gate electrode layer 160 b and a portion of the gate dielectric layer 150 b covered and protected by the second mask layer 180. The first dummy gate structures DGS1 and the second dummy gate structures DGS2 have substantially parallel longitudinal axes that are substantially perpendicular to longitudinal axes of the first fins 110 a and the second fins 110. After forming the first dummy gate structures DGS1 and the second dummy gate structures DGS2, the first mask layers 170 and the second mask layers 180 of FIG. 7 are removed. The first dummy gate structures DGS1 and the second dummy gate structures DGS2 will be replaced with replacement gate structures using a “gate-last” or replacement-gate process. Accordingly, the shape of the replacement gate structures can be determined by the shape of these dummy gate structures.

Further, because the width w₃₁ of at least one of the first mask layers 170 is less than the width w₃₂ of at least one of the second mask layers 180, a width (i.e., gate length) w₄₁ of at least one of the first dummy gate structures DGS1 is less than a width (i.e., gate length) w₄₂ of at least one of the second dummy gate structures DGS2. However, it is noted that the scope of this application is not limited thereto. The width w₄₁ and the width w₄₂ can be determined by the first mask layer 170 and the second mask layer 180 respectively. In some other embodiments, the width w₄₁ of at least one of the first dummy gate structures DGS1 is equal to the width w₄₂ of at least one of the second dummy gate structures DGS2. In still some embodiments, the width w₄₁ of at least one of the first dummy gate structures DGS1 is greater than the width w₄₂ of at least one of the second dummy gate structures DGS2.

Turning now to FIGS. 9A, 9B, and 9C to further clarify the instant disclosure, FIGS. 9A, 9B, and 9C are cross-sectional views along the lines A-A′, B-B′, and C-C′ in FIG. 8 respectively. As shown in FIGS. 9A-9C, the first height h₁ from the top of the first fin 110 a to the top of the first isolation dielectric 140 a is greater than the second height h₂ from the top of the second fin 110 b to the top of the second isolation dielectric 140 b. Moreover, the first isolation dielectric 140 a on the core region CR of the substrate 110 is thinner than the second isolation dielectric 140 b on the periphery region PR of the substrate 110.

Please refer to FIGS. 7 and 8 again. The dummy gate electrode layer 160 a and the gate dielectric layer 150 a are etched through the first mask layers 170 and the second mask layers 180 to form the first dummy gate structures DGS1 and the second dummy gate structures DGS2 until both the first isolation dielectric 140 a and the second isolation dielectric 140 b expose. It is noted that, because the first isolation dielectric 140 a is thinner than the second isolation dielectric 140 b, the second isolation dielectric 140 b is exposed earlier than the first isolation dielectric 140 a. When etching the dummy gate electrode layer 160 a and the gate dielectric layer 150 a to form the lower portions of the first dummy gate structures DGS1, lower portions of the second dummy gate structures DGS2 is continuously etched (or trimmed). Accordingly, the resulting second dummy gate structures DGS2 have lower portions tapering toward the substrate 110. In other words, footings of second dummy gate structures DGS2 have notched profile.

When a dummy gate structure is formed to cross over fins, if a pitch between two adjacent fins is small, it is not easy to form the dummy gate structure with predetermined shape, especially the footing of the dummy gate structure. One skilled in the art should understand that the footing profile of the dummy gate structure will influence the subsequent process of forming a replacement gate structure, and thus is a factor for breakdown voltage (VBD) performance. It is noted that, the footings of second dummy gate structures DGS2 of the instant disclosure have notched profile; therefore, the second dummy gate structures DGS2 of the instant disclosure have upper portions wider than lower portions. Therefore, after the second dummy gate structures DGS2 is removed to form gate trenches, the gate trenches have a good filling performance with replacement gate structures, reducing the possibility of breakdown between gate and source/drain.

Please refer to FIGS. 9B-9C. In FIG. 9B, it can be seen that the first dummy gate structure DGS1 has two sidewalls SW_(1a) substantially parallel to each other. However, in FIG. 9C, only a portion of the second dummy gate structure DGS2 has parallel sidewalls. More specifically, the second dummy gate structure DGS2 includes an upper portion DU having two first sidewalls SWU_(1b) substantially parallel to each other and a lower portion DL tapers toward the substrate 110. Moreover, a top width w₅₁ of the lower portion DL is greater than a bottom width w₅₂ of the lower portion DL. Based on the above, it can be seen that, because the thickness of the first isolation dielectric 140 a is less than the thickness of the second isolation dielectric 140 b, the first height h₁ from the top of the first fin 110 a to the top of the first isolation dielectric 140 a is greater than the second height h₂ from the top of the second fin 110 b to the top of the second isolation dielectric 140 b. Accordingly, dummy gate structures with different shapes can be formed concurrently in the core region CR and the periphery region PR respectively.

Still referring to FIG. 9C, the lower portion DL has second sidewalls SWL_(1b), and the second sidewalls SWL_(1b) are substantially straight. However, it is noted that the scope of this application is not limited thereto. In some other embodiments, the second sidewalls SWL_(1b) are concave. In FIG. 9C, the lower portion DL is below the top surface TS2 of the second fin 110 b. The lower portion DL has a top surface substantially leveling with the top surface TS2 of the second fins 110 b. However, it is noted that the scope of this application is not limited thereto. In some other embodiments, the lower portion DL has a top surface above the top surface TS2 of the second fin 110 b. In still some other embodiments, the lower portion DL has a top surface below the top surface TS2 of the second fin 110 b.

Attention is now invited to FIGS. 10 and 11A-11C. FIGS. 11A, 11B, and 11C are cross-sectional views along the lines A-A′, B-B′, and C-C′ in FIG. 10 respectively. First gate spacers 190 a are conformally formed on the opposite sidewalls SW_(1a) of first dummy gate structures DGS1, and second gate spacers 190 b are conformally formed on the opposite sidewalls SW_(1b), of second dummy gate structures DGS2. As shown in FIG. 11B, a pair of first gate spacers 190 a conformally covers sidewalls SW_(1a) of the first dummy gate structure DGS1. As shown FIG. 11C, a pair of second gate spacers 190 b conformally covers the first sidewalls SWU_(1b) of the upper portion DU of the second dummy gate structure DGS2 and second sidewalls SWL_(1b) of the lower portion DL of the second dummy gate structure DGS2. At least one of the second gate spacers 190 b has a slanted portion SP in contact with one of the second sidewalls SWL_(1b) of the lower portion DL of the second dummy gate structure DGS2.

In some embodiments, the first gate spacers 190 a and the second gate spacers 190 b may include silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, silicon carbonitride, silicon oxycarbonitride, silicon oxycarbide, porous dielectric materials, hydrogen doped silicon oxycarbide (SiOC:H), low-k dielectric materials or other suitable dielectric material. The first gate spacers 190 a and the second gate spacers 190 b may include a single layer or multilayer structure made of different dielectric materials. The method of forming the first gate spacers 190 a and the second gate spacers 190 b includes blanket forming a dielectric layer on the structure shown in FIG. 8 using, for example, CVD, PVD or ALD, and then performing an etching process such as anisotropic etching to remove horizontal portions of the dielectric layer. The remaining portions of the dielectric layer on the sidewalls SW_(1a) of the first dummy gate structures DGS1 can serve as the first gate spacers 190 a, and the remaining portions of the dielectric layer on the sidewalls SW _(1b) of the second dummy gate structures DGS2 can serve as the second gate spacers 190 b. In some embodiments, the first gate spacers 190 a and the second gate spacers 190 b may be used to offset subsequently formed doped regions, such as source/drain regions. The first gate spacers 190 a and the second gate spacers 190 b may further be used for designing or modifying the source/drain region profile.

Reference is made to FIGS. 12 and 13A-13C. FIGS. 13A, 13B, and 13C are cross-sectional views along the lines A-A′, B-B′, and C-C′ in FIG. 12 respectively. Portions of the first fins 110 a not covered by the first dummy gate structures DGS1 and first gate spacers 190 a, and portions of the second fins 110 b not covered by the second dummy gate structures DGS2 and the second gate spacers 190 b are respectively partially removed (or partially recessed) to form first recesses R1 and second recesses R2. After this removal, at least one of remaining first fins 110 a may have protruding portions 110 a ₁ and embedded portions 110 a ₂, and at least one of remaining second fins 110 b may have protruding portions 110 b ₁ and embedded portions 110 b ₂. The embedded portions 110 a ₂ are embedded in the first isolation dielectric 140 a, and exposed by the first recesses R1. The protruding portion 110 a ₁ protrudes from the embedded portions 110 a ₂ and is located between the first recesses R1. The embedded portions 110 b ₂ are embedded in the second isolation dielectric 140 b, and exposed by the second recesses R2. The protruding portion 110 b ₁ protrudes from the embedded portions 110 b ₂ and is located between the second recesses R2. The first dummy gate structures DGS1 wrap the protruding portions 110 a ₁ and the second dummy gate structures DGS2 wrap the protruding portions 110 b ₁, and hence the protruding portions 110 a ₁ and the protruding portions 110 b ₁ can act as channel regions of transistors. The embedded portions 110 a ₂ spaced apart from the first dummy gate structures DGS1 and the embedded portions 110 b ₂ spaced apart from the second dummy gate structures DGS2 can act as source/drain regions of transistors.

Formation of the first recesses R1 and the second recesses R2 may include a dry etching process, a wet etching process, or combination of dry and wet etching processes. This etching process may include reactive ion etch (RIE) using the first dummy gate structures DGS1, the first gate spacers 190 a, the second dummy gate structures DGS2, and the second gate spacers 190 b as masks, or by other suitable removal process.

Reference is made to FIGS. 14 and 15A-15C. FIGS. 15A, 15B, and 15C are cross-sectional views along the lines A-A′, B-B′, and C-C′ in FIG. 14 respectively. First epitaxial source/drain structures 200 a are respectively formed in the first recesses R1, and second epitaxial source/drain structures 200 b are respectively formed in the second recesses R2. The first epitaxial source/drain structures 200 a and the second epitaxial source/drain structures 200 b may be formed using one or more epitaxy or epitaxial (epi) processes, such that Si features, SiGe features, silicon phosphate (SiP) features, silicon carbide (SiC) features and/or other suitable features can be formed in a crystalline state on the embedded portions 110 a ₁ of the first fins 110 a and the embedded portions 110 b ₁ of the second fins 110 b respectively. As shown in FIGS. 15A and 15C, the second epitaxial source/drain structure 200 b is over one of the second fins 110 b, and has a footing portion FP in contact with the slanted portion SP of at least one of the second gate spacers 190 b. In some embodiments, lattice constants of the first epitaxial source/drain structures 200 a and the second epitaxial source/drain structures 200 b are different from that of the first fins 110 a and the second fins 110 b, so that the channel region between the first epitaxial source/drain structures 200 a can be strained or stressed by the first epitaxial source/drain structures 200 a, the channel region between the second epitaxial source/drain structures 200 b can be strained or stressed by the second epitaxial source/drain structures 200 b to improve carrier mobility of the semiconductor structure and enhance the performance of the semiconductor structure.

The epitaxy processes include CVD deposition techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy, and/or other suitable processes. The epitaxy process may use gaseous and/or liquid precursors, which interact with the composition of the semiconductor fin 150 (e.g., silicon, silicon germanium, silicon phosphate, or the like). The first epitaxial source/drain structures 200 a and the second epitaxial source/drain structures 200 b may be in-situ doped. The doping species include P-type dopants, such as boron or BF₂; N-type dopants, such as phosphorus or arsenic; and/or other suitable dopants including combinations thereof. If the first epitaxial source/drain structures 200 a and the second epitaxial source/drain structures 200 b are not in-situ doped, a second implantation process (i.e., a junction implant process) is performed to dope the first epitaxial source/drain structures 200 a and the second epitaxial source/drain structures 200 b. One or more annealing processes may be performed to activate the first epitaxial source/drain structures 200 a and the second epitaxial source/drain structures 200 b. The annealing processes include rapid thermal annealing (RTA) and/or laser annealing processes.

Reference is made to FIGS. 16 and 17A-17C. FIGS. 17A, 17B, and 17C are cross-sectional views along the lines A-A′, B-B′, and C-C′ in FIG. 16 respectively. A contact etch stop layer (CESL) 210 is blanket formed on the structure shown in FIG. 14, and an interlayer dielectric (ILD) layer 220 is formed on the CESL 210. A CMP process may be optionally performed to remove excessive material of the ILD layer 220 and the CESL 210 to expose the first dummy gate structures DGS1 and the second dummy gate structures DGS2. The CMP process may planarize a top surface of the ILD layer 220 with top surfaces of the first dummy gate structures DGS1, the first gate spacers 190 a, the second dummy gate structures DGS2, and the second gate spacers 190 b, and the CESL 210 in some embodiments. The CESL 210 includes silicon nitride, silicon oxynitride or other suitable materials. The CESL 210 can be formed using, for example, plasma enhanced CVD, low pressure CVD, ALD or other suitable techniques. The ILD layer 220 may include a material different from the CESL 210. In some embodiments, the ILD layer 220 may include silicon oxide, silicon nitride, silicon oxynitride, tetraethoxysilane (TEOS), phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), low-k dielectric material, and/or other suitable dielectric materials. Examples of low-k dielectric materials include, but are not limited to, fluorinated silica glass (FSG), carbon doped silicon oxide, amorphous fluorinated carbon, parylene, bis-benzocyclobutenes (BCB), or polyimide. The ILD layer 220 may be formed using, for example, CVD, ALD, spin-on-glass (SOG) or other suitable techniques.

Please refer to FIGS. 18 and 20. The first dummy gate structures DGS1 are replaced with first gate structures GS1, and the second dummy gate structures DGS2 are replaced with second gate structures GS2. As such, as shown in FIG. 20, a semiconductor structure 100 is formed. The semiconductor structure 100 may be intermediate structures fabricated during processing of an integrated circuit, or portion thereof, that may include static random access memory (SRAM) and/or other logic circuits, passive components such as resistors, capacitors, and inductors, and active components such as P-channel field effect transistors (PFET), N-channel FET (NFET), metal-oxide semiconductor field effect transistors (MOSFET), complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, other memory cells, and combinations thereof.

Attention is now invited to FIGS. 18 and 19A-19C. FIGS. 19A, 19B, and 19C are cross-sectional views along the lines A-A′, B-B′, and C-C′ in FIG. 18 respectively. The first dummy gate structures DGS1 is removed to form gate trenches GT1 with the first gate spacers 190 a as its sidewalls, and the second dummy gate structures DGS2 is removed to form gate trenches GT2 with the second gate spacers 190 b as its sidewalls. As shown in FIG. 19B, the exposed sidewalls of the first gate spacers 190 a are substantially straight. Therefore, the gate trench GT1 has substantially straight sidewalls. Further, a portion of the protruding portion 110 a ₁ of the first fin 110 a is exposed. As shown in FIG. 19C, the gate trench GT2 has an upper trench portion UP and a lower trench portion LP in communication with each other. The upper trench portion UP has substantially straight sidewalls, and the sidewalls substantially parallel to each other. The lower trench portion LP tapers toward the second isolation dielectric 140 b (or the periphery region PR of the substrate 110). In FIG. 19C, the sidewalls of the lower trench portion LP are substantially straight. However, it is noted that the scope of this application is not limited thereto. In some other embodiments, the sidewalls of the lower trench portion LP are convex. Further, a portion of the protruding portion 110 b ₁ of the second fin 110 b is exposed. In FIG. 19C, the lower trench portion LP of the gate trench GT2 is below the top surface TS2 of the second fin 110 b. The lower trench portion LP has a top substantially leveling with the top surface TS2 of the second fins 110 b. However, it is noted that the scope of this application is not limited thereto. In some other embodiments, the lower trench portion LP has a top above the top surface TS2 of the second fin 110 b. In still some other embodiments, the lower trench portion LP has a top below the top surface TS2 of the second fin 110 b.

In some embodiments, the first dummy gate structures DGS1 and the second dummy gate structures DGS2 are removed by performing a first etching process and performing a second etching process after the first etching process. In some embodiments, the dummy gate electrode layers 160 a, 160 b are mainly removed by the first etching process, and the gate dielectric layers 150 a, 150 b are mainly removed by the second etching process. In some embodiments, the first etching process is a dry etching process and the second etching process is a wet etching process. In some embodiments, the dry etching process includes using an etching gas such as CF₄, Ar, NF₃, C1 ₂, He, HBr, O₂, N₂, CH₃F, CH₄, CH₂F₂, or combinations thereof. In some embodiments, the dry etching process is performed at a temperature in a range from about 20° C. to about 80° C. In some embodiments, the dry etching process is performed at a pressure in a range from about 1 mTorr to about 100 mTorr. In some embodiments, the dry etching process is performed at a power in a range from about 50 W to about 1500 W.

Attention is now invited to FIGS. 20 and 21A-21C. FIGS. 21A, 21B, and 21C are cross-sectional views along the lines A-A′, B-B′, and C-C′ in FIG. 20 respectively. FIGS. 20 and 21A-21C illustrate formation of replacement gates. The first gate structures GS1 and the second gate structure GS2 are respectively formed in the gate trenches GT1 and GT2. Accordingly, the first gate structures GS1 crosses over the first fins 110 a, and the second gate structures GS2 crosses over the second fins 110 b. Exemplary method of forming these gate structures may include the following operations. A gate dielectric layer including a gate dielectric layer 230 a and a gate dielectric layer 230 b is blanket formed in the gate trenches GT1 and the gate trenches GT2, and over the first gate spacers 190 a, the second gate spacers 190 b, CESL 210, and ILD layer 220. A work function conductor including a work function conductor 240 a and a work function conductor 240 b is conformally formed over the gate dielectric layer. A layer of filling conductor including a filling conductor 250 a and a filling conductor 250 b is conformally formed over the work function conductor. A CMP process is performed to remove excessive materials of the gate dielectric layer 230 a, 230 b, the work function conductor 240 a, 240 b, and the filling conductor 250 a, 250 b outside the gate trenches GT1 and GT2. The resulting structure is shown in FIG. 20. In some embodiments, the work function conductor 240 a is a multi-layer structure. In some embodiments, the work function conductor 240 b is a multi-layer structure.

In some embodiments, the gate dielectric layer 230 a, 230 b may respectively include, for example, a high-k dielectric material such as metal oxides, metal nitrides, metal silicates, transition metal-oxides, transition metal-nitrides, transition metal-silicates, oxynitrides of metals, metal aluminates, zirconium silicate, zirconium aluminate, or combinations thereof. In some embodiments, the gate dielectric layer 230 a, 230 b may respectively include hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide (HMO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), lanthanum oxide (LaO), zirconium oxide (ZrO), titanium oxide (TiO), tantalum oxide (Ta₂O₅), yttrium oxide (Y₂O₃), strontium titanium oxide (SrTiO₃, STO), barium titanium oxide (BaTiO₃, BTO), barium zirconium oxide (BaZrO), hafnium lanthanum oxide (HfLaO), lanthanum silicon oxide (LaSiO), aluminum silicon oxide (AlSiO), aluminum oxide (A1 ₂O₃), silicon nitride (Si₃N₄), oxynitrides (SiON), and combinations thereof. In alternative embodiments, the gate dielectric layer 230 a, 230 b may respectively have a multilayer structure such as one layer of silicon oxide (e.g., interfacial layer) and another layer of high-k material. The formation of the gate dielectric layer 230 a, 230 b may include molecular-beam deposition (MBD), atomic layer deposition (ALD), physical vapor deposition (PVD), chemical vapor deposition (CVD), or the like.

The work function conductors 240 a, 240 b over the gate dielectric layers 230 a, 230 b respectively include work function metals to provide a suitable work function for the gate structures GS1, GS2. In some embodiments, the work function conductors 240 a, 240 b may respectively include one or more n-type work function metals (N-metal) for forming an n-type transistor on the substrate 110. The n-type work function metals may exemplarily include, but are not limited to, titanium aluminide (TiAl), titanium aluminium nitride (TiAlN), carbo-nitride tantalum (TaCN), hafnium (Hf), zirconium (Zr), titanium (Ti), tantalum (Ta), aluminum (Al), metal carbides (e.g., hafnium carbide (HfC), zirconium carbide (ZrC), titanium carbide (TiC), aluminum carbide (AlC)), aluminides, and/or other suitable materials. In alternative embodiments, the work function conductors 240 a, 240 b may respectively include one or more p-type work function metals (P-metal) for forming a p-type transistor on the substrate 110. The p-type work function metals may exemplarily include, but are not limited to, titanium nitride (TiN), tungsten nitride (WN), tungsten (W), ruthenium (Ru), palladium (Pd), platinum (Pt), cobalt (Co), nickel (Ni), conductive metal oxides, and/or other suitable materials.

The filling conductors 250 a, 250 b may exemplarily respectively include, but are not limited to, tungsten, aluminum, copper, nickel, cobalt, titanium, tantalum, titanium nitride, tantalum nitride, nickel silicide, cobalt silicide, TaC, TaSiN, TaCN, TiAl, TiAlN, or other suitable materials.

Still referring to FIG. 20, at least one of the first gate structures GS1 has a first gate length gl₁. At least one of the second gate structures GS2 has a second gate length g1 ₂. The first gate length gl₁ is less than the second gate length g1 ₂. However, it is noted that the scope of this application is not limited thereto. In some other embodiments, the first gate length gl₁ is greater than the second gate length g1 ₂. In still some other embodiments, the first gate length gl₁ is substantially equal to the second gate length g1 ₂.

As shown in FIGS. 20 and 21A-21C, the semiconductor structure 100 includes a substrate 110, the first gate structures GS1, the second gate structures GS2, the first gate spacers 190 a, the second gate spacers 190 b, the contact etch stop layer 210, the interlayer dielectric layer 220, the first epitaxial source/drain structures 200 a, and the second epitaxial source/drain structures 200 b. The substrate 110 has the core region CR and the periphery region PR adjacent to the core region CR. The core region CR of substrate 110 has the first fins 110 a. The periphery region PR has the second fins 110 b. As shown in FIG. 21A, the first pitch p₂₁ between two adjacent first fins 110 a is greater than the second pitch p₂₂ between two adjacent second fins 110 b. Moreover, the first isolation dielectric 140 a is between the first fins 110 a. The second isolation dielectric 140 b is between the second fins 110 b. The first isolation dielectric 140 a is thinner than the second isolation dielectric 140 b.

As shown in FIG. 20, at least one of the first gate structures GS1 crosses over the first fins and includes the gate dielectric layer 230 a, the work function conductor 240 a, and the filling conductor 250 a. As shown in FIG. 21B, the first gate structure GS1 has two sidewalls SW_(2a) substantially parallel to each other. A pair of the first gate spacers 190 a conformally covers the sidewalls SW_(2a) of the first gate structure GS1. As shown in FIG. 20, each second gate structure GS2 crosses over the second fins 110 b, and includes the gate dielectric layer 230 b, the work function conductor 240 b, and the filling conductor 250 b. As shown in FIG. 21C, the second gate structure GS2 includes an upper portion GU and a lower portion GL, and has two sidewalls SW_(2b). Each sidewall SW_(2b) includes first sidewalls SWU_(2b) and second sidewalls SWL_(2b). The upper portion GU has two first sidewalls SWU_(2b) substantially parallel to each other, and a lower portion GL tapers toward the periphery region PR of the substrate 110 (or the second isolation dielectric 140 b). A top width w₆₁ of the lower portion GL is greater than a bottom width w₆₂ of the lower portion GL. In FIG. 21C, the lower portion GL has two second sidewalls SWL_(2b), and the second sidewalls SWL_(2b) are substantially straight. However, it is noted that the scope of this application is not limited thereto. In some other embodiments, the second sidewalls SWL_(2b) are concave. A pair of the second gate spacers 190 b conformally covers the first sidewalls SWU_(2b) of the upper portion GU and the second sidewalls SWL_(2b) of the lower portion GL. At least one of the second gate spacers 190 b has the slanted portion SP in contact with one of the sidewalls SWL_(2b) of the lower portion GL of the second gate structure GS2.

Still referring to FIG. 20, the pairs of the first gate spacers 190 a cross over the first fins 110 a. In each pair of the first gate spacers 190 a, the gate trench GT1 is between two first gate spacers 190 a. As shown in 21B, the gate trench GT1 has sidewalls substantially parallel to each other. The first gate structure GS1 is disposed in the gate trench GT1. More specifically, the gate dielectric layer 230 a of the first gate structure GS1 is conformally disposed in the gate trench GT1 and covers the sidewalls and bottom surface of the gate trench GT1. The work function conductor 240 a of the first gate structure GS1 is conformally disposed on the gate dielectric layer 230 a. The filling conductor 250 a of the first gate structure GS1 is conformally disposed on the work function conductor 240 a.

Still referring to FIG. 20, the pairs of the second gate spacers 190 a cross over the second fins 110 b. In each pair of the second gate spacers 190 a, the gate trench GT2 is between two second gate spacers 190 a. As shown in 21C, the gate trench GT2 has the upper trench portion UP and the lower trench portion LP in communication with each other. The upper trench portion UP has substantially straight sidewalls, and the sidewalls substantially parallel to each other. The lower trench portion LP tapers toward the second isolation dielectric 140 b (or the periphery region PR of the substrate 110). The second gate structure GS2 is disposed in the gate trench GT2. More specifically, the gate dielectric layer 230 b of the second gate structure GS2 is conformally disposed in the gate trench GT2, and covers the sidewalls of the upper trench portion UP of the gate trench GT2 and the sidewalls and bottom surface of the lower trench portion LP of the gate trench GT2. The work function conductor 240 b of the second gate structure GS2 is conformally disposed on the gate dielectric layer 230 b. The filling conductor 250 b of the second gate structure GS2 is conformally disposed on the work function conductor 240 b.

Please refer to FIGS. 19C and 21C simultaneously. As previously described in FIG. 19C, the lower trench portion LP of the gate trench GT2 is below the top surface TS2 of the second fin 110 b. The lower trench portion LP has the top substantially leveling with the top surface TS2 of the second fins 110 b. Therefore, the lower portion GL of the second gate structure GS2 (i.e., the portion of the second gate structure GS2 filled in the lower trench portion LP) is below the top surface TS2 of the second fin 110 b. The lower portion GL of the second gate structures GS2 has a top surface substantially leveling with the top surface TS2 of the second fin 110 b. However, it is noted that the scope of this application is not limited thereto. In some other embodiments, the lower portion GL of the second gate structures GS2 has a top surface above the top surface TS2 of the second fin 110 b. In still some other embodiments, the lower portion GL of the second gate structures GS2 has a top surface below the top surface TS2 of the second fin 110 b.

Embodiments of the instant disclosure may have at least following advantages. The dummy gate structures on the periphery region of the substrate have upper portions wider than lower portions. Accordingly, when replacing the dummy gate structures with the gate structures, the shape of gate trenches formed after removing the dummy gate structures may be advantageous to fill the gate structures and enlarge breakdown voltage (VBD) between gate structures and source/drain.

In some embodiments of the instant disclosure, a semiconductor structure includes a substrate, a first gate structure, and a second gate structure. The substrate has a plurality of first fins and a plurality of second fins, wherein a first pitch between two adjacent first fins is greater than a second pitch between two adjacent second fins. The first gate structure crosses over the first fins. The second gate structure crosses over the second fins, wherein the second gate structure includes an upper portion having two first sidewalls substantially parallel to each other and a lower portion tapers toward the substrate.

In some embodiments of the instant disclosure, a semiconductor structure includes a substrate, a first isolation dielectric, a second isolation dielectric, a first gate structure, and a second gate structure. The substrate has a core region and a periphery region, wherein the core region has a plurality of first fins, the periphery region has a plurality of second fins, and the first fins and the second fins have substantially the same height. The first isolation dielectric is between the first fins. The second isolation dielectric is between the second fins, wherein the first isolation dielectric is thinner than the second isolation dielectric. The first gate structure crosses over the first fins. The second gate structure crosses over the second fins. The first gate structure has a first gate length, the second gate structure has a second gate length, and the first gate length is less than the second gate length.

In some embodiments of the instant disclosure, a method of manufacturing a semiconductor structure includes forming a substrate having a plurality of first fins and a plurality of second fins, wherein a first pitch between two adjacent first fins is greater than a second pitch between two adjacent second fins, forming an isolation dielectric covering the first fins and the second fins, reducing a thickness of the isolation dielectric to leave the isolation dielectric covering top surfaces of the first fins and the second fins, etching the isolation dielectric covering the top surfaces of the first fins and the second fins to form a first isolation dielectric between the first fins and a second isolation dielectric between the second fins, and forming a first gate structure crossing over the first fins and a second gate structure crossing over the second fins.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

1. A semiconductor structure, comprising: a substrate having a plurality of first fins and a plurality of second fins, wherein a first pitch between two adjacent first fins is greater than a second pitch between two adjacent second fins; a first gate structure crossing over the first fins; and a second gate structure crossing over the second fins, wherein the second gate structure comprises an upper portion having two first sidewalls substantially parallel to each other and a lower portion having two second sidewalls tapering toward the substrate, wherein the lower portion of the second gate structure has a top width greater than a width of the first gate structure, wherein the top width is a longest width between the second sidewalls.
 2. The semiconductor structure of claim 1, further comprising a first isolation dielectric between the first fins and a second isolation dielectric between the second fins, wherein a first height from a top of at least one of the first fins to a top of the first isolation dielectric is greater than a second height from a top of at least one of the second fins to a top of the second isolation dielectric.
 3. The semiconductor structure of claim 1, wherein the lower portion of the second gate structure is below top surfaces of the second fins.
 4. (canceled)
 5. The semiconductor structure of claim 1, further comprising a pair of gate spacers conformally covering the first sidewalls of the upper portion and the second sidewalls of the lower portion of the second gate structure.
 6. The semiconductor structure of claim 5, wherein at least one of the gate spacers has a slanted portion in contact with one of the second sidewalls of the lower portion of the second gate structure, and the semiconductor structure further comprises an epitaxial source/drain structure over one of the second fins, wherein the epitaxial source/drain structure has a footing portion in contact with the slanted portion.
 7. The semiconductor structure of claim 1, wherein the first gate structure has two sidewalls substantially parallel to each other.
 8. A semiconductor structure, comprising: a substrate having a core region and a periphery region, wherein the core region has a plurality of first fins, the periphery region has a plurality of second fins, and the first fins and the second fins have substantially the same height; a first isolation dielectric between the first fins; a second isolation dielectric between the second fins, wherein the first isolation dielectric is thinner than the second isolation dielectric; a first gate structure crossing over the first fins; and a second gate structure crossing over the second fins, wherein the first and second gate structures respectively have first and second gate lengths along a direction parallel to a longitudinal axis of one of the first fins, and the first gate length is less than the second gate length.
 9. The semiconductor structure of claim 8, wherein a first height from a top of at least one of the first fins to a top of the first isolation dielectric is greater than a second height from a top of at least one of the second fins to a top of the second isolation dielectric.
 10. The semiconductor structure of claim 8, wherein the second gate structure has an upper portion and a lower portion, and a top width of the lower portion is greater than a bottom width of the lower portion.
 11. The semiconductor structure of claim 10, wherein the lower portion of the second gate structure has a top surface substantially leveling with top surfaces of the second fins.
 12. The semiconductor structure of claim 10, wherein the upper portion of the second gate structure has two sidewalls parallel to each other.
 13. The semiconductor structure of claim 10, further comprising a pair of gate spacers conformally covering sidewalls of the upper portion and the lower portion of the second gate structure. 14-20. (canceled)
 21. A semiconductor structure, comprising: first and second isolation dielectrics on a substrate; a first fin extending a first height above a top surface of the first isolation dielectric; a second fin extending a second height above a top surface of the second isolation dielectric, wherein the second height is different from the first height; and first and second gate structures respectively extending across the first and second fins, wherein cross sections of the first and second gate structures that are cut along a direction parallel to a longitudinal axis of the first fin have different shapes.
 22. The semiconductor structure of claim 21, wherein the second gate structure comprises a first portion and a second portion atop the first portion, and the first and second portions have different cross-sectional profiles.
 23. The semiconductor structure of claim 22, wherein the first portion tapers towards the substrate.
 24. The semiconductor structure of claim 22, wherein the second height is less than the first height.
 25. The semiconductor structure of claim 21, further comprising: first and second gate spacers respectively abutting the first and second gate structures and having different cross-sectional profiles.
 26. The semiconductor structure of claim 25, wherein the second gate spacer comprises a first spacer portion and a second spacer portion atop the first spacer portion, and the first and second spacer portions have different cross-sectional profiles.
 27. The semiconductor structure of claim 26, wherein the first spacer portion is slanted relative to the top surface of the second isolation dielectric.
 28. The semiconductor structure of claim 7, wherein one of the sidewalls of the first gate structure has a different slope than one of the second sidewalls of the lower portion of the second gate structure. 